In my last post, I managed to deploy a Compojure app to Google App Engine. Serving static content isn’t very exciting, though. Pretty much every app will need some way to store and retrieve data. So let’s try to access the App Engine Datastore.

New Dependencies

To use the datastore API I need to include a jar that comes with the GAE SDK in my app. I could call the Java API directly, but there are already a few Clojure libraries that provide friendly wrappers around it. One of the first (that I know of) was appengine-clj by John Hume (who was also one of the first to write about using Clojure on GAE).

I decided to go with this fork of appengine-clj by Roman Scherer, which seems to be more complete and actively maintained. Another interesting option would be clj-gae-datastore by the people at freiheit.com.

I updated my project.clj file with the new dependencies:

I’m using Hiccup (formerly part of Compojure) for HTML generation. Running lein deps runs into an error, because it can’t find the Google SDK jar in the public repositories. Luckily, Leiningen (or Maven) already tells me how to fix this by installing the jar from the SDK download into my local Maven repository – I just have to copy and paste the command from the error message and enter the path to the local jar. After that, lein deps executes cleanly and copies the new dependencies into my lib dir.

Along with updating the dependencies in project.clj I have to import the needed symbols into my namespace:

The choice of using :use or :require is pretty arbitrary in this case – I just used both to demonstrate the different options. With :require I need to call the functions using the full namespace (using a short alias), with :use they are imported into my namespace. For the latter case, I explicitly named all the definitions I need using :only. This is pretty verbose and not strictly necessary, but for a little howto like this I want you to immediately see where every function comes from, so you don’t have to rummage through all the libraries (although the doc function makes this easy…). I guess it generally is a good idea to not clutter your namespace with definitions you don’t need.

Storing Data

Now comes the more interesting part: Actually accessing the datastore. I want to be able to create simple blog posts, consisting of a title and a body. I need two new routes, one for displaying a form, and one that is used as the action URL for the form:

Here’s the code that handles the form submission:

Amazingly simple. The create-entity function just takes a Clojure map, which needs to have a “:kind” entry, and stores it in the datastore. After that I issue an HTTP redirect to the main page.

Retrieving Data

Retrieving data is just as simple. On the main page, I just display all posts:

The h function takes care of escaping special characters in the user input, so I don’t run into any cross-site scripting trouble. render-page is a little helper function that takes care of constructing the common HTML around the payload for all pages.

As usual, the whole code can be found at Github. The version as of this writing is here.

Basic Security

I don’t want the whole world to be able to post to my blog, so I need some authentication and authorization. I could use the App Engine Users API, but I’ll leave that for a later post. Instead I’ll go the simple route and enable security for some URLs in the deployment descriptor. That way the application itself is blissfully unaware of it. I just need to add this to the web.xml file:

Now only logged-in admin users can post new entries. You can see the deployed version of the app here: http://v0-2.latest.compojureongae.appspot.com/

What about the REPL!?

Okay, everything works fine. I can compile the project, start the dev_appserver to test it locally and deploy it to the Google cloud (see my last post for the steps). But what about interactive development? When I try to call e.g. the create-entity function from a REPL, I only get an Exception. So I can develop and deploy working software, but I’m back to the dreaded edit-compile-run cycle – that’s not the Clojure way.

I need to fix this, but it’ll have to wait until the next post. Sorry…